1. Field of Invention
The present invention relates to a communication technology in signal processing. More particularly, the present invention relates to a method for recovering a digital data content in a communication system and an apparatus to perform the method.
2. Description of Related Art
Nowadays, the wireless communication has been a very usual way taken by people for communication between two places without much restriction in space and distance. In addition, since the digital data technology has also been well developed, most of the wireless communications now have been in a digital manner. In digital communication, the digital data content at the transmitting end is converted into a radio frequency (RF) analog signal for transmission. After receiving the RF analog signal, the receiving end performs the conversion of the RF analog signal to the digital data content.
At the transmitting end, the digital data content is first converted into digital signal according to the applied coding scheme, modulation scheme, and etc. The digital signal is then converted by a digital-to-analog converter (DAC) to an analog signal according to a local clock 102 shown in FIG. 1. The analog signal is then further processed, for example, shifting the central frequency of the analog signal to a radio frequency band, for transmission. At the receiving end, the received RF analog signal is first processed, for example, reducing the central frequency of the RF analog signal to a much lower one. The analog signal is then converted into a digital signal by an analog-to-digital converter (ADC), which samples the analog signal in time domain based on a local clock 104 shown in FIG. 1, and quantizes the sampled signal amplitude for further digital signal process (DSP).
However, the clock rates between the transmitting and receiving ends are generally different, that is, the clock periods of the clock 102 at transmitting end and the clock 104 at the receiving end are generally different, and accordingly cause serious degradation of DSP performance at the receiving end. As an example shown in FIG. 1, the timing drift, which is the time distance between the ticks of the clock 102 and the clock 104, increases linearly due to the mismatch of the clock rates. Usually, the time drift will cause an accumulative divergence of synchronization between two timing device or sources. When the timing drift is accumulated up to a certain quantity, it could cause serious errors in data recovery.
Traditionally, one solution to timing drift problem may concentrate on increasing the timing resolution via increasing the clock rate of the clock 104 at the receiving end and accordingly increasing the ADC sampling rate. The receiver can then select those sampled digital signal at more precise sampling position so that the timing drift are controlled to be less than the ADC sampling period, which is defined to be the inverse of the ADC sampling rate. However, the use of a high-sampling-rate ADC increases not only the cost but also the complexity of the design. Although the use of a low-sampling-rate ADC is a proper way to reduce the cost as well as the complexity of the design, the timing resolution is decreased. Moreover, due to the decrease of the timing resolution, the timing drift problem gets more serious and the system performance is then degraded.
Alternatively, an interpolator may be used to accept the output of the ADC and generate an output, called interpolated digital signal, with increased timing accuracy by interpolating the input signal at the desired time point, named interpolation points, so that the timing of the interpolated digital signal coincides with that of the clock at transmitting end. According to the Nyquist sampling theorem, which is commonly familiar to those skilled artisans in the art, the original analog signal waveform can be exactly reconstructed according to its sampled signal if the sampling rate is higher than a Nyquist rate, which is defined to be equal to twice of the signal spectrum bandwidth. Otherwise, signal distortion will occur so that the original analog signal will not be recovered exactly. Hence, theoretically, an ADC with sampling rate higher than the Nyquist rate plus an ideal interpolator is sufficient to generate digital signals having any required timing accuracy without signal distortion.
The usage of a simplified interpolator, which equipped with a small number of the interpolation filter coefficients, to replace the ideal interpolator, which would need a large number of the interpolation filter coefficients, can further decrease the complexity of the interpolator. An example of the simplified interpolator is a simplified linear interpolator that employs only two coefficients to compute the signal at a specified time point between those of two consecutive input signals. However, the simplified interpolator will cause signal distortion due to the small number of interpolation filter coefficients. And the system performance degrades.